1. Field of the Invention
The present invention relates to a switching regulator for outputting a constant voltage, and more particularly, to a switching regulator including a short-circuit detection circuit.
2. Description of the Related Art
Switching regulators are used as a voltage supply source for a circuit in various kinds of electronic devices. The function of the switching regulator is to output a constant voltage to an output terminal irrespective of fluctuations in input terminal voltage. It is important to operate a short-circuit protection circuit for detecting that some abnormality has occurred in the output terminal and a current supplied to a load has increased to exceed a maximum current (see, for example, Japanese Patent Application Laid-open No. Hei 5-328711).
FIG. 3 illustrates a circuit diagram of a conventional switching regulator including a short-circuit detection circuit. The conventional switching regulator includes an output voltage dividing circuit 20 for dividing a voltage of an output terminal Out, a reference voltage circuit 4 for outputting a reference voltage Vref, an error amplifier 1 for comparing a divided voltage FB to the reference voltage Vref, a triangle wave oscillation circuit 3 for outputting a triangle wave signal Vramp, a PWM comparator circuit 2 for comparing an output voltage of the error amplifier 1 to the triangle wave signal Vramp and outputting a pulse signal, a buffer circuit 5 for amplifying the output signal of the PWM comparator 2, a short-circuit detection circuit 100 for detecting the flow of an overcurrent to the output terminal Out, a power switching element 200, an inductance 201, a diode 202, and a smoothing capacitor 203. The short-circuit detection circuit 100 includes a second reference voltage circuit 102 and a comparator circuit 101.
The operation of the conventional switching regulator is described below. The conventional switching regulator operates under PWM control as shown in the figures.
The triangle wave signal Vramp output from the triangle wave oscillation circuit 3 has a constant frequency. The duty cycle of a signal Vpwm output from the PWM comparator 2 changes in accordance with increase and decrease in load current. The signal Vpwm controls a conduction time of the power switching element 200. Then, an output voltage Vout of the output terminal Out is maintained to a constant value.
FIG. 4 is a timing chart of the conventional switching regulator. Under a condition where the load current is constant as shown in a period t1, the duty cycle of the signal Vpwm is constant. When the load current increases as shown in a period t2, a voltage Verr output from the error amplifier 1 decreases, and the duty cycle of the signal Vpwm increases. When the load current further increases, and the voltage Verr falls below an amplitude lower limit of the triangle wave signal Vramp, the signal Vpwm continues to output a fixed value for bringing the power switching element 200 into the conductive state all the time.
In this case, a reference voltage Vref2 of the second reference voltage circuit 102 is set to the lower voltage side than the amplitude lower limit of the triangle wave signal Vramp. When the output terminal Out reaches the short-circuit state as shown in a period t3, the voltage Verr further decreases. When the voltage Verr falls below the reference voltage Vref2, the short-circuit detection circuit 100 detects the short-circuit state, and a voltage SD output from the comparator circuit 101 is inverted to L level. The buffer circuit 5 is turned OFF in response to the voltage SD of L level, thereby bringing the power switching element 200 into the non-conductive state. In this way, the short-circuit detection circuit 100 prevents the switching regulator from continuing to allow an overcurrent to flow.
However, in the case of using the circuit for comparing the output voltage Verr of the error amplifier to the reference voltage Vref2, it is necessary to set the reference voltage Vref2 to be out of the amplitude range of the triangle wave signal Vramp and within the amplitude range of the output voltage of the error amplifier. This requires an operation of adjusting the reference voltage Vref2, such as trimming, thus leading to the disadvantage of increased cost.